Matching networks are used in order to transform the impedance provided by the load of an amplifier, e.g. a plasma processing system, to a proper load impedance at a reference plane of the amplifying device, e.g. a transistor. Especially matching networks which can be integrated on a printed circuit board (PCB) are interesting in order to enable cost-effective construction and beneficial form factors. For the implementation of such matching networks at operation frequencies below 100 MHz planar transformers are very important. The matching bandwidth of such planar matching networks is usually low. The efficiency and output power of the combination of the amplifying device and the matching network therefore strongly depends on the impedance deviation with respect to the intended operation frequency.